Not Applicable.
Pain sensation is mediated by nerve fibers. Nerve fibers extend to the brain via the spinal cord which forms a portion of the central nervous system. Referring to FIG. 1, the spinal cord 12 extends from the brain to the level of the second lumbar vertebra, at which point the spinal cord branches to numerous individual roots. Throughout the length of the spinal cord, the same is encased in the vertebral canal. Nerves 14 branch off at regular intervals.
A number of types of nerves are disposed within the posterior gray horn 16. Two types of pain sensing nerves have been identified: Axcex4 and C. Referring to FIGS. 2 and 3, Axcex4 fibers 18 are disposed within regions I and V and the same produce a rapid initial and intense response to painful stimuli. C fibers 20 are disposed within region II and produce a more blunted but prolonged response. C fibers 20 are believed to be responsible for many chronic pain syndromes.
Axcex4 fibers 18 and C fibers 20 are connected to the spinal cord 20 via the dorsal root 22 (referring back to FIG. 1). The dorsal root 22 is a bundle of nerves that enters the dorsal aspect of the spinal cord 12. The sensory nerves from one particular body region, such as the right leg, may be split among several dorsal root nerve bundles spaced along the length of the spinal cord 12.
Pain is conducted via fibers of the peripheral nervous system to the central nervous system, or nerves in the spinal cord. The pain signal is conducted up nerve tracts of the spinal cord to the pain sensing areas of the brain (i.e., the thalamus). The transmission of the pain signal from the peripheral nerves to the central nerves takes place in the synapses of the posterior gray horn region 16 of the spinal cord 12. A synapse is a neuron-to-neuron transmission of a signal by a chemical mediator that traverses a small gap between two axon terminals.
As noted above, many Axcex4 fibers 18 and C fibers 20 synapse in the most superficial, or dorsal, region of the dorsal gray horn known as zones 1 and 2. The synaptic region of the C fibers 20 is also known as the substantia gelatinosa. Various treatments directed at these fibers and these anatomical locations, can be and are used to treat pain syndromes.
An estimated 15 million Americans suffer from chronic intractable pain. 50% of persons with terminal illness have significant pain and 10% require a surgical procedure to treat the pain. $80 billion is spent annually in the United States-on chronic pain.
Current therapy for chronic pain can be divided into two categories: medical and surgical. Medical therapy is the administration of drugs ranging from Tylenol(copyright) to morphine. Morphine and its analogs are used in cases of severe pain and terminal illness. These drugs have many serious side effects such as sedation, confusion, constipation, and depression of respiration. The more severe the pain, the higher the dosage of the drug and the more significant the side effects. In addition, tolerance to these compounds develops, and escalating doses are required to achieve pain control.
Surgical therapy can range from the implantation of drug infusion devices to the ablation, or destruction, of nerves. Ablation of nervous tissue is irreversible and can cause permanent loss of function of organs and limbs. One type of surgical treatment is known as Dorsal Root Entry Zone (xe2x80x9cDREZxe2x80x9d) ablation. The DREZ is shown in FIG. 1 as DREZ 24. While DREZ ablation is effective at treating pain, it can also result in significant limb and organ dysfunction. Drug infusion into the spinal cord using implanted devices can reduce drug side effects, however they do not eliminate side effects entirely nor solve the problem of tolerance. These approaches require significant surgical procedures; often, terminally ill patients are not good candidates for surgery.
Nerve stimulators are also used for pain control. These electrical devices work indirectly by stimulating nerve fibers that inhibit conduction pain fibers. It is known to place devices such as nerve stimulators surgically or percutaneously and they may be placed directly adjacent to the spinal cord. For example, U.S. Pat. No. 5,643,330 to Holsheimer et al., issued Jul. 1, 1997, and entitled xe2x80x9cMultichannel Apparatus for Epidural Spinal Cord Stimulationxe2x80x9d, discloses placing an epidural spinal cord stimulator adjacent to spinal cord dura mater.
Stimulators are relatively ineffective in controlling pain. This may be in part due to the indirect mechanism of action. Further, they can cause dysthesias and paresthesias (neurologenic pain) due to the stimulation of nerve fibers.
There is a need for a method and apparatus to combat pain, especially chronic pain, which do not suffer from the drawbacks of current medical and surgical therapies.
In one aspect, the invention is directed to a method of cooling a portion of a spinal cord of a patient. The method includes delivering a portion of a heat pipe to a spinal cord of a patient, the heat pipe including an evaporator and a condenser, including disposing the evaporator at least in partial thermal communication with the spinal cord. The evaporator is cooled by passing a working fluid between the evaporator and the condenser.
Implementations of the invention may include one or more of the following. The delivering may further include disposing the evaporator at least in partial thermal communication with the dorsal root entry zone of the spinal cord. The working fluid may be passed between the evaporator and the condenser through a conduit, and the conduit may be a tube or wick structure, for example. The condenser may be implanted within a patient or may be located externally of a patient. The condenser may have an insulated lower chamber into which the conduit enters and an upper chamber into which the return tube enters, the lower and upper chambers separated by a porous structure, and may further include passing the working fluid in gaseous form from the evaporator through the return tube within the conduit to the upper chamber, condensing the working fluid at least partially from the gaseous form into the liquid form, passing the working fluid from the upper chamber to the lower chamber through the porous structure, and passing the condensed working fluid from the lower chamber to the evaporator through the conduit. Another implementation may include disposing the upper chamber in thermal communication with a cold source. The evaporator may be disposed adjacent the dura mater, or between the spinal cord and the dura mater, or on the side of the dura mater opposite the spinal cord.
In another aspect, the invention is directed to an apparatus for cooling a portion of tissue. The apparatus includes an evaporator to be placed in thermal communication with a portion of tissue; a condenser disposed in thermal communication with a source or sink of heat, the condenser including an upper chamber and a lower chamber; and a conduit disposed between the evaporator and the condenser, the conduit including a wick structure, to communicate working fluid between the two. An implementation of the invention may include providing a porous structure to separate the lower chamber from the upper chamber.
Advantages of the invention include one or more of the following. The invention provides for control of chronic pain in an effective manner. The processes used to achieve hypothermia to control pain are reversible. Nerve tissue is not destroyed as in certain other techniques. Nerve tissue recovers completely when the processes are stopped. The invention allows for treatment of not only chronic pain but also urge incontinence, muscle spasticity, epilepsy, and may even be of benefit in treating multiple sclerosis.